Generally, an active matrix organic light emitting diode (AMOLED) in a flat panel display, as shown in FIG. 1, includes a pixel array portion 110 where a plurality of pixels are arranged on an insulating substrate 100 in the form of matrix, and a driving circuit portion for driving the pixel array portion 110. The pixel array portion 110 includes a plurality of gate lines, a plurality of data lines, a plurality of common power lines, and a plurality of pixels connected to these lines, in the form of matrix (not shown in FIG. 1). Each of pixels comprises an electroluminesence (EL) device, a driving transistor for supplying a driving current in accordance with a data signal from the data line to the EL device, a switching transistor for transferring the data signal to the driving transistor in response to a scanning signal applied to the gate line, and a capacitor for storing the data signal.
The driving circuit portion for driving the pixels of the pixel array portion 110 comprises a gate driving circuit portion 130 for supplying the scanning signal for driving the gate line of the pixel array portion 110, and a data driving circuit portion 120 for supplying the data signal to the data line of the pixel array portion 110.
In a conventional AMOLED, all of thin film transistors of the pixel array portion 110 and thin film transistors of the driving circuit portions 120 and 130 consist of polysilicon thin film transistors. However, in an AMOLED having a 180 ppi resolution or higher where the pixel array portion and the driving circuit portion consist of polysilicon thin film transistors (poly-TFTs), a high speed operating characteristic of the driving circuit portion could be achieved from high mobility of the poly-TFTs. However, the on-current of the poly-TFT is extremely high so that the amount of current flowing through the EL device of the pixel array portion exceeds the limit value, thereby increasing the luminance per unit area and shortening the lifetime of the EL device.
Meanwhile, where the pixel array portion and the driving circuit portion consist of TFTs having a lower mobility to maintain the on-current characteristic at a required level, the on-current becomes relatively low in the pixel array portion so that the proper luminance is generated, thereby solving the lifetime problem of the EL device. However, the high speed-operating characteristic of the driving circuit portion are not satisfied.